1. Field of the Invention
This invention relates to apparatus and methods for elemental analysis, in particular to pre-treatment of a sample to be analysed.
2. Description of the Prior Art
Elemental analysis encompasses a variety of techniques which involve the qualitative and/or quantitative determination of the elemental composition of a sample based on the atomic properties of the constituent elements.
One such form of elemental analysis is atomic emission spectroscopy (AES), which involves exciting the chemical elements in a sample to emit light of one or more specific wavelengths (spectral light). Atomisation and excitation of the sample occurs in a suitable source, for example, an inductively coupled plasma (ICP). The technique is conventionally applied to a fine dispersion of the sample under investigation. The dispersion may be prepared by various means; spray generators, thermospray systems, pneumatic nebulisers or ultrasonic nebulisers (USNs) may be used in the case of liquid samples; and electric arc heating or laser ablation used in the case of solid samples.
By liquid sample is meant any sample for analysis in which the analyte(s) are present in a liquid carrier, for example, solutions, dispersions, suspensions, colloids and slurries.
In practice it is found that the quality of the analysis depends strongly on the particle size distribution of the dispersion, the presence of larger particles being deleterious in that the atomisation and excitation of such larger particles is less efficient.
In the case of liquid samples it is also desirable that the liquid carrier is evaporated from the dispersed droplets so that the analyte enters the plasma source as an essentially dry aerosol. Evaporation of the liquid is advantageous because it cancels the cooling effect on the plasma caused by the injection of large volumes of liquid, meaning more energy is available for atomisation and excitation; it also reduces other carrier effects such as plasma instability and spectral interferences, for example, oxide interferences when the carrier is water.
Conventional methods of evaporation involve passing the dispersed sample, in a flow of carrier gas, through a heated tube, at for example, 130.degree. C., to evaporate the liquid carrier and then through a refrigerated tube to condense and separate the liquid from the analyte aerosol. In such a process the sample is heated, by conduction, to the boiling point of the droplet over a period of about one second, liquid evaporates leaving the analyte contained therein as a single particle. The resulting particle size is therefore defined by the droplet size and the concentration of analyte in the sample. Thus, even with the fine dispersions produced by, for example, USNs the analyte particles left after evaporation of the liquid may still be larger than is desirable for efficient atomisation and excitation.